1. Field of the Invention.
This invention relates to an electrically-conductive aramid pulp composition that has high surface area, a high concentration of fibrils and increases strength and high modulus as polymeric reinforcement.
2. Description of Related Art.
U.S. Pat. Nos. 5,788,897 and 5,882,566, issued Aug. 4, 1998 and Mar. 16, 1999, respectively, disclose fibers having a continuous phase of para-aramid and a discontinuous phase of electrically-conductive sulfonated polyaniline.
U.S. Pat. No. 5,094,913, issued Mar. 10, 1992, discloses a pulp refined from fibers having a continuous phase of para-aramid and a discontinuous phase of meta-aramid.
Japanese Patent Publication (Kokai) No. 59/163418, published Sep. 14, 1984, discloses pulp beaten from fibers of a blend of para-aramid and aliphatic polyamide.
This invention includes a composition in the form of a pulp comprising a blend of 65 to 95 weight percent of para-aramid and 5 to 35 weight percent of sulfonated polyaniline (SPA) wherein the para-aramid is present in the composition as a continuous phase and the SPA is dispersed throughout the para-aramid. Pulp particles in the composition generally have a specific surface area of greater than 7.5 m2/g and a Canadian Standard Freeness of less than 150 milliliters.
Paper made from the pulp of this invention exhibits a charge decay rate of less than 5 seconds.
Electrically conductive pulp is a very desirable product for use in reinforcement of packaging films and polymers, generally, and especially where there is a need to drain or dissipate electrical charges. Electrically conductive pulp finds use in applications where handling dielectric pulp, in dry form, results in charged particles that are difficult to handle or are dangerous due to a threat of sparking on discharge.
This invention utilizes an intimate blend of two polymeric materials to provide a pulp that is not only a good reinforcement for other polymers but is, also, electrically conductive to impart electrical conductivity to normally dielectric materials into which it is added for reinforcement. Fibers of combined polymers are known. Particularly fibers of para-aramid combined with other polymersxe2x80x94and even polyaniline polymersxe2x80x94are known. However, there has been, up to now, no suggestion that such fibers might be refined to make conductive pulp materials.
This invention provides a pulp product that is not only an excellent reinforcement material, is also extremely effective for electric charge dissipation. Moreover, the very material good for such charge dissipation is the material that creates ease in pulp manufacture and excellence in pulp quality.
The materials of this pulp product are para-aramid and SPA and the SPA component provides a dual function with the purposes widely divergent and largely unrelated. First, the polyaniline, as a secondary component in the blend, provides points of fracture for refining and pulping forces to achieve efficient and effective manufacture of high quality pulp with fine, long, fibrils. Second, the polyaniline, as a component effectively on the surface of the pulp particles, provides an electrical conductivity that is effective in dissipating electrical charge by contact of the fibrils on adjacent pulp particles.
By xe2x80x9caramidxe2x80x9d is meant a polyamide wherein at least 85% of the amide (xe2x80x94COxe2x80x94NHxe2x80x94) linkages are attached directly to two aromatic rings. Aramid fibers are described in Man-Made Fibersxe2x80x94Science and Technology, Volume 2, Section titled Fiber-Forming Aromatic Polyamides, page 297, W. Black et al., Interscience Publishers, 1968. Aramid fibers are, also, disclosed in U.S. Pat. Nos. 4,172,938; 3,869,429; 3,819,587; 3,673,143; 3,354,127; and 3,094,511.
Para-aramids are the primary polymers of this invention for blending with polyaniline; and poly(p-phenylene terephthalamide) is the preferred para-aramid. By para-aramid is meant the homopolymer resulting from mole-for-mole polymerization of para-phenylene diamine and terephthaloyl chloride and, also, copolymers resulting from incorporation of small amounts of other diamines with the para-phenylene diamine and of small amounts of other diacid chlorides with the terephthaloyl chloride. As a general rule, other diamines and other diacid chlorides can be used in amounts up to as much as about 30 mole percent of the para-phenylene diamine or the terephthaloyl chloride, or perhaps slightly higher, provided only that the other diamines and diacid chlorides have no reactive groups that interfere with the polymerization reaction. Para-aramid, also, means copolymers resulting from incorporation of other aromatic diamines and other aromatic diacid chlorides such as, for example, 2,6-naphthaloyl chloride or chloro- or dichloroterephthaloyl chloride; provided, only that the other aromatic diamines and aromatic diacid chlorides be present in amounts which permit preparation of anisotropic spin dopes. Preparation of para-aramids and processes for spinning fibers from the para-aramids are described in U.S. Pat. Nos. 3,869,429; 4,308,374; 4,698,414; and 5,459,231.
Sulfonated polyaniline of the present invention can be made by in-situ ring-sulfonation. The term xe2x80x9cinsitu ring-sulfonationxe2x80x9d means that the polyaniline is sulfonated during the polymer solutioning process and not isolated from the sulfuric acid solution before the solution is spun into a fiber. Of course, the sulfonation can, also, be achieved in any other way to make sulfonated polyaniline leading to a conductive pulp.
To be effective in practice of this invention, the sulfonated polyaniline must be sulfonated to a degree that will provide adequate conductivity to drain electrical charges. It has been found that sulfonation is required to a sulfur content of at least 8.5 percent, based on total weight of the sulfonated polyaniline. Sulfonation of less than that amount, results in generally inadequate fiber conductivity. It has, also, been found that increased sulfonation yields improved performance up to a sulfonation level of about 15 weight percent sulfur, based on total weight of the sulfonated polyaniline. Sulfonation to a greater degree has been found to be of little additional benefit. It is noted that sulfonation of polyaniline, to a degree of 8.5 to 15 weight percent, corresponds to a mol percent sulfonation of about 30 to 70 percent of the polyaniline repeat units.
The pulp of this invention can be made by so-called air gap spinning of anisotropic spin dope including the para-aramid and the sulfonated polyaniline. Preparation of such spin dope and spinning of fibers to serve as the basis for the pulp used in this invention, can be found in aforementioned U.S. Pat. Nos. 5,788,897 and 5,882,566.
The molecular weight of the polyaniline employed in the pulp of this invention is not critical. Polyaniline of low molecular weight may result in lower solution viscosity and easier processing, however, it might be more readily removed from the fiber in processing or use.
High molecular weight para-aramid is usedxe2x80x94having an inherent viscosity of at least 5. In order to obtain pulp of the desirable high strength and modulus, a spin dope concentration of the para-aramid is employed that results in an anisotropic dope as discussed in U.S. Pat. No. 3,767,756. Spin dopes containing at least 13% by wt. of total polymer content, that is, sulfonated polyaniline plus the p-aramid, meet this requirement. Otherwise the mechanical properties of the spun fiber will not be acceptable for preparation of the pulp to provide antistatic properties.
The concentration of sulfonated polyaniline in para-aramid in the spin solution, and ultimately in the spun fiber and the pulp product, has an important influence on properties. As the content of sulfonated polyaniline increases to and exceeds 40 wt % of the polymer mixture, the tensile strength of the fiber becomes undesirably reduced with no concomitant increase in electrical conductivity. Also, in washing fibers with such a high concentration of polyaniline, some of the insitu ring-sulfonated polyaniline may be extracted.
The ring-sulfonated polyaniline should constitute at least 3 weight percent and preferably more than 5 weight percent of the pulp product to provide a charge decay rate of less than about 5 seconds. The ring-sulfonated polyaniline should constitute from 3 to 40 weight percent and preferably from 5 to 30 weight percent of the fibers, based on the polymer mixture with calculations using unsulfonated polyaniline.
To make the pulp of this invention, fibers that have been spun as described above, are cut into uniform lengths of 0.5 to 2.5 cm and are suspended in water to form a floc that is subjected to high shear conditions to produce pulp. Equipment useful for refining cellulosic fibers, such as refiners having abrading elements that rotate relative to one another, is useful for this purpose. In pulping in accordance with this invention, shearing along boundaries between the para-aramid and polyaniline phases results readily in the formation of high quality pulp particles with excellent pulp length and high degree of fibrillation. The presence of the polyaniline domains provides fracture points in the chopped fiber and leads to ready and more complete fibrillation at reduced energy consumption, wherein pulp particle surfaces are, at least in part, defined by the location of polyaniline domains running through the fibers. As a result of that definition, at least some of the outer surfaces of the pulp have a relatively high concentration of polyaniline and an unexpectedly high electrical conductivity.
One reliable indicator of the degree of fibrillation and the level of surface area of a pulp product is known as xe2x80x9cCanadian Standard Freenessxe2x80x9d (CSF). The CSF of a pulp is reported as a volume of drained water determined as a result of a specified testing procedure explained herein below. Pulp eligible for use in the instant invention generally exhibits a CSF of 0 to 150 ml and preferably 20 to 100 ml. Lower CSF is generally some indication of higher surface area.
The composition of this invention may include a pulp blend combination of the two-component pulp and pulp made from other material. In that case, the composition need only contain as much of the two-component pulp as is required to achieve the desired charge decay rate. Compositions exhibiting a charge decay rate of less than five seconds are within the bounds of this invention. The amount of two-component pulp required to achieve such a charge decay rate varies depending on the amount of sulfur in the sulfonated polyaniline and the amount of sulfonated polyaniline in the two-component pulp. As a general matter, pulp blend compositions must have at least 5 weight percent two-component pulp and less than 95 weight percent of the other pulp material, based on the total weight of the composition.
The pulp component made from other material can be made from any other pulpable material including, for example, cellulosic material, acrylics, para-aramids, and the like. The preferred other pulp material is the para-aramid material, poly(p-phenylene terephthalamide).
Electric Charge Dissipation
The static decay or electric charge dissipation test measures the ability of a material, when grounded, to dissipate a known charge that has been induced on the surface of the material. To test electric charge dissipation of the pulps made in these examples, pulp was made into paper sheets and charge dissipation tests were conducted on the sheets.
Five grams of a pulp were dispersed for five minutes in 1.5 liters of water in a TMI disperser (Testing Machines, Inc., Islandia, N.Y.). The resulting slurry was poured into the head box of a laboratory handsheet machine containing 25 liters of water. A handsheet 30xc3x9730 cm was formed, dewatered, and dried.
Static Decay Rate test specimens, 9xc3x9714 cm, were cut from the handsheets, equilibrated for at least 24 hours at 30% relative humidity, and tested using an ETS Static Decay Meter, Model 406C (Electro-Tech Systems, Inc.).
In conduct of the test, the test specimens are mounted between electrodes of the Meter, a charge of 5000 volts is applied, and, on grounding the electrodes, the time is measured for the charge to drain to 500 volts. This test is Federal Test Method Standard 101B, Method 4046, known as the Static Decay Test. Test results are set out in Table IV.
Sulfur Content
A pulp sample of known weight is combusted with oxygen in a flask; and the generated SO2 and SO3 gases are absorbed in water. Hydrogen peroxide is added to the water to insure that all sulfur is converted to sulfate; and the water is boiled with platinum black to remove any excess H2O2. The resulting solution is combined with an equal volume of isopropanol and titrated with a standardized BaCl2 solution for determination of sulfate concentration. The amount of sulfur is determined based on the sulfate concentration.
Pulp Length
Pulp fiber length is measured using a Kajaani FS-200 instrument (Kajaani Electronics, Kajaani, Finland). An aqueous slurry of pulp fibers is prepared at a concentration adequate for a rate of analysis of 40xcx9c60 fibers per second. The slurry is passed through the capillary of the instrument for exposure to a laser beam and a detector to determine the fiber length. The instrument performs calculations from the detector output and reports three different lengths;xe2x80x94the arithmetic average length, the length-weighted average length; and the weight-weighted average length.
Tensile Properties
Filaments tested for tensile properties are, first, conditioned at 25xc2x0 C., 55% relative humidity for a minimum of 14 hours; and the tensile tests are conducted at those conditions. Tenacity (breaking tenacity), elongation (breaking elongation), and modulus are determined by breaking test filaments on an Instron tester (Instron Engineering Corp., Canton, Mass.).
Tenacity, elongation, and initial modulus, as defined in ASTM D2101-1985, are determined using filament gage lengths of 2.54 cm. Tenacity is reported in grams per denier. The modulus is calculated from the slope of the stress-strain curve at 1% strain and is equal to the stress in grams at 1% strain (absolute) times 100, divided by the test filament denier. Filament denier is determined according to ASTM D1577 using a vibrascope.
Specific Surface Area
Surface areas are determined utilizing a single point BET nitrogen absorption method using a Strohlein surface area meter (Standard Instrumentation, Inc., Charleston, W.Va.). Washed samples of pulp are dried in a tared sample flask, weighed and placed on the apparatus. Nitrogen is adsorbed at liquid nitrogen temperature. Adsorption is measured by the pressure difference between sample and reference flasks (manometer readings) and specific surface area is calculated from the manometer readings, the barometric pressure, and the sample weight.
Canadian Standard Freeness
This is a measure of the drainage of a suspension of 3 grams of fibrous material in 1 liter of water. Measurement and apparatus are according to TAPPI Standard T227 om-94. The fibrous material is dispersed for five minutes in a TMI disperser. Results are reported as volume (ml) of water drained under standard conditions. The measured value is affected by the fineness and flexibility of the fibers and by their degree of fibrillation.